Method for controlling the feed of nuclei in the production of crystals by vapor deposition



Dec. 24, 1968 w. a. CAMPBELL 3,413,076

METHOD FOR CONTROLLING THE FEED OF NUCLEI IN THE PRODUCTION OF CRYSTALS BY VAPOR DEPOSITION Filed April 14. 1965 2 Sheets-Sheet 1 .J g g\ 2 I (L l- 3 m a: 3 5 1 0 3 a: g g o (I) O r O Q & Q m 5 a Q INVENTOR. 3 WILLIAM B. CAMPBELL BY yaldoaw ATTORNEY Dec. 24, 1968 w. B. CAMPBELL METHOD FOR CONTROLLING THE FEED OF NUCLEI IN THE PRODUCTION OF CRYSTALS BY VAPOR DEPOSITION Filed April 14. 1965 2 Sheets-Sheet 2 L L R E I] 1| mm v m N m p U 00+ oo w B N M a m i i m T 3v R m M w E H N 0 h QM Qw Q8 wx m 9w v w M 3 w i I 3R? .QQQQ; k H xiii): 1 a HM ink X A I: V E F. i W T United States Patent 3,418,076 METHOD FOR CONTROLLING THE FEED OF NUCLEI IN THE PRODUCTION OF CRYS- TALS BY VAPOR DEPOSITION William B. Campbell, Belmont, Mass., assignor to Lexington Laboratories Inc., Cambridge, Mass., a corporation of Massachusetts Filed Apr. 14, 1965, Ser. No. 447,994 8 Claims. (Cl. 23-142) ABSTRACT OF THE DISCLOSURE An improved method of delivering solid nuclei to the reaction zone where single crystal whiskers are grown on the nuclei by deposition from the vapor phase. The nuclei are removed from a supply station and conveyed as a stream along a tube by mechanical vibrations which constantly agitate the nuclei and control the density of the stream. A carrier gas is injected into the stream at a point remote from the supply station to produce a mixture consisting of carrier gas and nuclei which is fed to the reaction zone.

This invention relates to the synthesis of single crystals and more particularly to improved process and apparatus for growing single crystals by vapor deposition.

It is well known that single crystals may be grown epitaxially on a substrate by deposition involving interaction of vapor phase reactants at the solid-vapor interface. Under controlled conditions of pressure, temperature, and supersaturation, it is possible to grow crystals in the form of fine fibers (also called whiskers) characterized by high purity and crystalline perfection. Preferably, the single crystal fibers are grown on small nuclei of the same stoichiometric composition which are fed into the reaction zone at the same time as the vapor phase reactants. Although various compounds may be grown in whisker form, there has been considerable difilculty in doing so on a continuous or semi-continuous basis with consistency of results. While for the most part the inconsistency of results obtained by others is due to improper choice of reaction conditions, I have determined that it also is due in part to failure to feed the nuclei into the reaction zone at a substantially constant rate. In this connection, it is to be appreciated that precise control of supersaturation is essential in order to achieve proper growth and that precise control of supersaturation cannot be attained unless a steady reaction state exists in the reaction zone. However, steady state operation is determined by the equilibrium equation:

where P partial pressure of the product gases and P partial pressure of the reactant gases. The nuclei have a partial pressure with evaporation occurring at their surfaces. If the rate of nuclei delivery to the reaction zone varies, the rate of nuclei evaporation and the pressures in the growth region also will vary, thereby affecting the equation so as to drive the reaction in the forward or reverse direction according to the nature of the pressure change. Depending upon the extent of the change in rate of nuclei delivery, the alfect on deposition from the vapor phase may vary from a mere change in crystal growth velocity to a change in growth form, e.g., a powder. Of course, this effect also will vary in degree with different crystal compositions. The desired control over rate of nuclei delivery is not easily achieved. For example, I have determined that it is not satisfactory to deliver nuclei from a fluidized bed or via conventional aspirator-type injectors. A fluidized bed comprising particles of a size suitable for the process tends to pulse and v the rate of removal of nuclei from the bed to the reaction zone is not steady but instead is modulated more or less in accordance with the pulsing of the bed. Conventional aspirator-type injectors are not satisfactory because of cost as well as inadequate flow control.

Accordingly, the primary object of my present invention is to provide an improved method of growing single crystal -fibers involving a novel mode of delivering nuclei to the crystal growth region at a controlled rate.

A more particular object of the invention is to provide an improved method of delivering selected nuclei to a reaction zone Where single crystal fibers are grown on said nuclei by deposition from the vapor phase.

Another particular object is to provide novel apparatus for delivering nuclei to a reaction zone where single crys tals are grown thereon by deposition from the vapor phase.

A further object of the invention is to produce crystal fibers of high purity and perfection at controlled growth rate.

Other objects and many of the attendant advantages of my invention will become more readily apparent as reference is had to the following detailed specification when considered together with the acompanying drawings wherein:

FIG. 1 schematically illustrates a preferred system for growing sapphire whiskers in accordance with the present invention;

FIG. 2 shows a preferred form of nuclei feeder embodying the present invention; and

FIG. 3 is a cross-sectional view taken along line 33 of FIG. 2.

The invention is described in detail hereinafter as applied to the production of sapphire whiskers. However, such description is intended to be merely illustrative and it is to be understood that the principles of my process and apparatus are applicable to the production of crystals of different composition.

The system shown in FIG. 1 is adapted to accomplish vapor phase growth of sapphire whiskers according to a preferred process based upon the following reaction equation:

The system comprises a reaction chamber in the form of a closed furnace 2 which is heated by suitable means such as an electric resistance coil 4. At its inlet end, the furnace is connected to an inlet line 6 that is fed by two supply lines 8 and 10. The former leads to a source 12 of highest purity aluminum chloride (AlCl gas and separate sources 16 and 18 of highly pure chlorine and hydrogen. Line 10 leads to a source 20 of nuclei and separate sources 22 and 24 of highly pure carbon dioxide and carbon monoxide. The nuclei consists of tit-alumina particles whose size is in the order of microns. The nuclei and CO CO mixture are brought together by means of the apparatus of FIG. 2, whereupon the nuclei become entrained in the gas mixture and are delivered by the latter to the furnace. Although not shown, it is to be understood that control valves, flow meters, and pressure gauges are provided at appropriate points to permit individual regulation of the flow of each of the dive gases AlCl C1 H CO, and CO Electrical resistance heating coils such as coil 26 are provided to preheat the gases before they enter the furnace.

At its downstream end, the furnace has an outlet line 28 which is connected via a crystal separator 30 to a vacuum pump 32. The latter is adapted to withdraw gases and whiskers from the furnace while maintaining the overall reaction chamber pressure at a desired level. Crystal separator is essentially a filtering device adapted to pass the gaseous efiiuent from the reaction chamber while retaining the whisker-bearing nuclei.

The temperature within the furnace is kept constant at a selected level at which whisker growth will occur according to the foregoing overall reaction equation under given flow and pressure conditions. Thermocouples (not shown) may be used to determine the temperature within the furnace as well as the temperature of the gases as they enter the reaction chamber. Suitable means (also not shown) are employed to monitor the pressure within the furnace at all times during whisker production.

Turning now to FIG. 2, the apparatus for achieving controlled delivery of nuclei to the reaction chamber comprises a base 34 supporting an electrically powered vibrator unit 36 that is of conventional design and has an output shaft 38 that reciprocates at a suit-able frequency, e.g.,

600 cycles per second. Although not shown, it is to be understood that the vibrator unit includes means for adjusting its output amplitude, i.e., the length of the stroke of shaft 38. Attached to the base in inverted position is a generally U-shaped support member 40 consisting of a pair of inclined legs 42 and 44 connected at one end by an intermediate horizontal bar 46. Shaft 38 is connected to leg 42. The two legs 42 and 44 are relatively stiff and have just enough resiliency to flex in response to reciprocal movement of shaft 38. As a consequence of the geometry of the foregoing arrangement, the intermediate bar 46 will oscillate along a path of movement characterized by a relatively large horizontal vector and a relatively small vertical vector, i.e., at an acute angle to the plane of bar 46.

Mounted on bar 46 of support member 40 is an assembly comprising an elongated housing in the form of a tube 48 which preferably is made up of several sections that are releasably secured together in a suitable manner. By way of example, the illustrated embodiment comprises three sections having mating fianges 50 that are secured together by a plurality of bolts 52 and nuts 54. This arrangement facilitates disassembly for inspection and cleaning purposes. The tube may be secured to the U-shaped support member in any conventional manner, such as by one or more friction bands 56 or by welding or cementing. The exact manner in which tube 48 and its vibratory support member 40 are united is not critical except that the two members must vibrate as one assembly. It also is preferred that the connection between support member 40 and tube 48 be at or adjacent to one end of the tube as shown.

Both ends of tube 48 are closed by end walls. However, at the end that is attached to support member 40, the top side of tube 48 is provided with a relatively large inlet hole in which is mounted an upstanding tubular chute 60 for introduction of nuclei 62. The chute is provided with a removable cover 64. At a suitable point downstream of chute 60, the tube 48 has another inlet hole 66 fitted with a pipe 68 for admittance of a carrier gas. In the illustrated embodiment, pipe 68 is connected to the CO and CO gas sources 22 and 24 respectively. Inlet 66 is located on the underside of tube 48 at the six oclock position. Mounted within tube 48 intermediate chute 60 and inlet 66 is a bafile plate 70 provided with an opening 72 of suitable size at the six oclock position. Preferably, the baffle plate is located close to inlet 66 as shown.

At its opposite end, the tube 48 is fitted with two outlet pipes 74 and 76 located at the twelve and six oclock positions, respectively. Pipe 74 leads to the furnace 2 and corresponds to line 10 in FIG. 1. Pipe 76 carries a removable container 78 at its bottom end.

Operation of the apparatus of FIG. 2 will be readily understood from the following example illustrating how a-alumina whiskers may be grown with and according to the present invention:

Example Chute 60 is filled with a charge of alumina nuclei having a size in the range of 5-10 microns, and cover 64 is secured in place. Furnace 2 is heated to about 1560 C.

and the gas supply lines are heated to a level such that the various gases will be at a temperature of 300-400" C. as they enter the reaction chamber provided by furnace 2. At the same time, the system is pumped down to a vacuum of about 50 microns of mercury. Then vibrator unit 36 is started and gas flow is initiated with hydrogen flow commencing last. The flow rates of the various gases in liters per minute are as follows: aluminum chloride- 0.03; hydrogen0.95; chlorine0.02; carbon dioxide 0.80; and CO0.22. Thereafter, the pressure within the reaction chamber is adjusted to 5.5 mm. of mercury and is maintained at that level during the run. The foregoing flow rates and temperatures also are held constant during the run.

Under the influence of vibrator unit 36 acting through shaft 38 and support member 40', tube 48 will oscillate steadily in a generally longitudinal direction. This vibratory movement causes the alumina nuclei to migrate away from the stockpile at chute 60 toward bafile plate 70. The migrating nuclei form a continuous stream in which each particle is constantly agitated. The stream rapidly shrinks in width as it moves away from the stockpile. This shrinkage in width is due at least in part to the geometry of tube 48, the particles tending to assume the six oclock position in the tube. The density of the stream is controlled by the operating level of the vibrator unit. The stream is at its minimum width and has a substantially constant density by the time it reaches and passes through the hole 72 in baffle plate 70. The latter prevents turbulent backflow of the CO CO gas mixture toward chute 60 and thus prevents any substantial disturbance of the nuclei stream until after it passes through hole 72. As the nuclei move past the baffle plate, they are swept up by and become suspended in the incoming stream of CO CO gas mixture. The nuclei are swept up as fast as they move downstream of the baffie plate so that the concentration thereof in the CO -CO gas delivered to furnace 2 remains constant during the run.

Under the foregoing operating conditions, alumina whiskers form on the nuclei in furnace 2. These whiskerbearing nuclei are collected in the crystal separator during the run which is terminated after the supply of nuclei has been consumed. Little growth occurs on the furnace walls, and the lines leading to and from the furnace are clean and free of crystal growth. The product recovered from crystal separator 30 consists of rhombohedral, prismatic, and hexagonal alpha-alumina whiskers grown on the injected nuclei. The whiskers have lengths up to inch and diameters in the order of about 7 microns. Their tensile strength measured atroom temperature is in the range of 1.8 to 2 million pounds per square inch.

The results achieved according to the foregoing example are typical and can be reproduced repetitively using the novel apparatus of FIG. 2. It has not been possible to achieve comparable consistency of results when delivering nuclei in accordance with prior art practices, e.g., from a fluidized bed or by means of conventional aspirator-type injection nozzle. Unlike prior art equipment, the rate of introduction of nuclei is not controlled wholly by the rate of flow of the carrier gas (i.e., the CO CO mixture in the foregoing example), but is variable according to the level of operation of the vibrator unit. Thus, within certain limits, the rate of delivery of nuclei can be adjusted Without modifying the rate of flow of the carrier gas. A further advantage resides in the fact that the carrier for the nuclei can be one or a mixture of the gases involved in the reaction (as in the foregoing example) or may be an inert gas such as argon. Another advantage is that nuclei that are too large to be swept out of tube 48 by the carrier gas cannot collect at a flow obstructing point but instead drop via pipe 76 into container 78 from which they can be removed at a convenient time.

Of course, the apparatus of FIG. 2 is applicable to the growth of whisker-form crystals of other substances, e.g., boron, which can be formed on nuclei by deposition from a vapor phase reaction. The manner in which the equipment is constructed and its mode of operation allows continuous operation without need for frequent interruptions to clean out lines, nozzles, etc.

It is to be understood also that the cross-sectional shape of the tube 48 need not be circular as shown but may conform to any other suitable configuration which tends to promote formation under the influence of gravity of a narrow stream of nuclei along its bottom side. Thus, tube 48 could have a triangular shape with its side walls converging at the bottom and the carrier gas inlet pipe 68 mounted close to the bottom edge of either side or else partially in both sides. Accordingly, it is to be understood that use of the terms underside or bottom side in connection with location of the nuclei stream in tube 48 (whether of circular or other suitable configuration) is intended to denote the low point in the tube, e.g., the six oclock position shown in FIG. 3. It is to be noted also that best results are obtained if the diameter of pipe 68 is such that the carrier gas stream has a width comparable to the stream of nuclei so as to assure that it is uniformly affected by the carrier gas.

Of course, the invention is not limited in its application to the details, arrangement, or example specifically described or illustrated, and within the scope of the appended claims, it may be practiced otherwise than as specifically described or illustrated.

I claim:

1. In a process for growing crystals on selected nuclei by deposition from a vapor phase reaction involving a plurality of selected reactants occurring in a reaction chamber, the improvement consisting of establishing a supply of solid nuclei whose size is in the order of microns, removing nuclei from said supply and conveying said removed nuclei as a thin constantly agitated stream along an elongate tube by mechanically vibrating said tube, injecting a carrier gas into said stream as it advances along said tube so as to remove nuclei therefrom and produce a mixture consisting of nuclei suspended in said carrier gas, and delivering said mixture to said reaction chamber.

2. A process as defined by claim 1 wherein said carrier gas is one of said selected reactants.

3. The method of claim 1 wherein said carrier gas is injected as a stream which is at least as wide as said stream of nuclei.

4. The method of claim 1 wherein said tube is substantially horizontal and said carrier gas is injected into the bottom side of said tube.

5. A process as defined by claim 1 wherein said vapor phase reaction produces A1 0 crystals and said reactants comprise AlCl C12, H2, CO2 and CO.

6. A process as defined by claim 5 wherein said nuclei are particles of alumina.

7. In a process for growing alumina crystals on selected solid nuclei by deposition from a vapor phase reaction occurring in a reaction chamber, the improvement consisting of establishing a supply of solid nuclei with a particle size in the order of microns adjacent one end of an elongate tube which extends in a generally horizontal direction, subjecting said tube to vmechanical vibrations so as to cause said nuclei to migrate away from said supply toward the opposite end of said tube in a thin free-flowing stream that is confined by gravity to the bottom side of said tube and moves at a substantially constant rate, injecting a carrier gas into said stream via an opening in the bottom side of said tube at a velocity sufiicient to remove nuclei from said stream and form a mixture consisting of nuclei suspended in said gas, and delivering said mixture to said reaction chamber at a controlled rate.

8. The invention of claim 7 wherein said vapor phase reaction involves AlCl C1 H CO and CO gases with at least one of said gases serving as the carrier for said nuclei.

References Cited UNITED STATES PATENTS 2,837,451 6/1958 Hannon 23-142 X 2,914,357 11/1959 Goering et al 302-66 3,065,032 11/ 1962 Sylvester 302-56 3,068,113 12/1962 Strain et al 10630O 3,094,385 6/1963 Brisbin et al. 23-142 3,148,027 9/1964 Richmond 23140 X 3,190,728 6/1965 Vunderink 23-273 EARL C. THOMAS, Primary Examiner.

G. T. OZAKI, Assistant Examiner.

US. Cl. X.R. 

